1. Field of the Invention
This invention relates to a stress measuring rock support device and, more particularly, to a mechanism for determining and measuring axial tensile load or strain on a cable bolt, friction stabilizer or similar means used for supporting rock strata.
2. Brief Description of the Prior Art
It is desirable in mining operations, for maximum extraction of ore at various depths, to mine blocks of ore of the largest possible dimensions. The limiting factor for the design is the strength of the rock in the vicinity of the ore body, which prior to mining may be inherently weak or highly fractured due to geological conditions. In situations where the strength of the ore body is exceeded, the implication for personnel safety as well as for "dilution" of the ore by the surrounding waste rock, loss of reserves, and the like, are very serious.
A universally accepted engineering solution to this problem is to reinforce the rock using steel cable bolts or dowels or similar devices cemented or forced into boreholes. The basic principle, much the same as that for reinforced concrete, is to provide the additional strength required to enable the rock to support itself. A good description of such rock bolts, dowels and the like, and of the manner in which they are used, is provided in the book entitled "Support of Underground Excavations in Hard Rock" by E. Hoek, P. K. Kaiser and W. F. Bawden, particularly at pp. 152-164, published by A. A. Balkema/Rotterdam/Brookfield in 1995.
However, as with any composite involving cement or concrete, the weakest link is the contact between the different components. For a cable bolt this corresponds to the cable/grout interface, and often results in failure due to slip at the cement-steel interface at loads to significantly less than that required to rupture the steel cable.
To counteract this problem, several recent innovations have involved the modification of the basic cable geometry by opening up the weave of the cable enough to allow the cement to flow between the individual wires, thus creating a stronger "mechanical" interlock between the cement and the cable. Such alternatives offer benefits and drawbacks with respect to performance, installation convenience and price. At present, the so called "bulge" and "nutcase" cables are modified geometry cable bolts that are often used in Canadian mines. A third modified cable, the Garford.TM. bulb anchor cable, is also being introduced into Canadian mines.
Modified cable geometries will provide more effective cable support in poor quality ground and under adverse mining conditions, however their widespread acceptance is dependent on further non-site specific evaluation and monitoring of performance. More importantly, if modified cable bolt designs are to be optimized, i.e. both as to pattern and length, instrumentation is required to directly and accurately monitor the loads that modified geometry cables are subject to under various in-situ conditions. Also, there will always be a significant use of conventional 7-wire stranded cable bolts and it is important to be able to adapt such cable bolt construction so that it may also be accurately and effectively monitored. Moreover, friction stabilizers, such as the Split Set.TM. stabilizer produced by Ingersoll-Rand, would also greatly benefit from accurate monitoring of the tensile load or strain to which they may be subjected.
The basic requirements for such instrumentation are as follows: (i) it should be easy to install even under adverse conditions; (ii) it should have adequate sensitivity and accuracy for the intended purpose, but also be able to perform under a range of expected displacements; (iii) it should be robust and be suitably protected to ensure durability for the required duration of operation; (iv) it should be easy to read and results should be immediately available to the operator; (v) it should not interfere with the effectiveness of the rock support means, such as the cable bolt or friction stabilizer, and (vi) it should be of low cost so as to be within the financial range of most operations.
Efforts have been made to produce such a monitoring gauge using a resistance wire concept. U.S. Pat. No. 4,803,888 of Feb. 14, 1989 describes one such resistance wire measuring gauge. However, the basic problem with resistance wire monitors, is that resistance wires, such as Nichrome, yield at about 0.5% strain, while the cable which they measure may yield at, for example, 0.8% strain. Thus, when the cable is under high load, the resistance wire starts to yield while the cable is still elastic and no accurate reading can be obtained under such conditions.
Another problem with such devices is that the resistance wires are attached to the exterior of the cable, i.e. at the cable-grout interface, which is the interface at which failure and slip occur as the cable is progressively loaded. Hence, results may be strongly affected by the presence of the instrument itself.
An accurate stress measuring rock support device would be especially beneficial for modified geometry cables because the objective there is to optimize the cable pattern, without overloading and rupturing the cable itself. With modified geometry cables, the principal mode of failure is usually the tensile rupture of the steel cable. Thus, rupture of such cables is predictable; it occurs at or around 25 tonnes axial load in a single strand cable and it is therefore important to know when such load is being approached.
As a matter of fact, a modified geometry cable design can be unsuccessful in two ways:
(1) if single strand cable loads exceed 25 tonnes, cable rupture results in immediate falls of ground, and PA0 (2) if cable loads are very low (e.g. 10-15 tonnes), the design is overconservative and hence unduly costly.
Furthermore, as mining proceeds, mining induced stress changes may result in delayed failures. The stress measuring instrument output may be used to gain a better understanding of stand-up time and to provide an indication of imminent failure. Such data are useful for risk assessment in critical areas (e.g. entry vs non-entry areas).